Active noise control (ANC) is a technique for reducing vehicle noise, and controls the noise of booming bandwidth having a frequency of 250 Hz or less by using an inverted phase sound wave to the noise. When a noise is generated, active noise control emits a sound wave having an inverted phase to the original noise through an audio apparatus, thereby canceling the noise.
In general, noise control methods can largely be divided into methods that can reduce noise emission from a noise source, methods that can block noise by using sound absorbing material, and methods that can cancel noise by using active noise control. Among these methods, the method of using sound absorbing material is effective with respect to a high frequency noise higher than 500 Hz. However, in the case of a low frequency noise, the size of the sound absorbing material needs to be suitably increased, which results in increased cost. Accordingly, when weight reduction is desirable, such as for an aircraft or a vehicle, the method may not be suitable.
Research has been conducted on active noise control that uses a secondary source, and it has been found that active noise control shows an excellent effect compared to conventional manual methods in canceling a low frequency noise, for example, a noise generated in a rotary machine, a transformer, or a car engine.
Research studies have been developed concerning noise control in a duct, an active noise control in three-dimensional space such as a plant or an office, the design of a noise barrier using the active noise control, and an active noise control in a vehicle or an airplane. More recently, due to increasing environmental awareness, there has been an increasing demand for a car that consumes less fuel so as to reduce the emission of exhaust gases. Accordingly, research has been initiated directed to reducing the weight of a car while, at the same time, increasing engine efficiency. However, so far, this approach has resulted in increased noise and vibration in the car.
At the same time, there is a demand from customers for more comfortable and quiet cars, and active noise control has been proposed as a solution to such demand due to its ability to actively control the interior noise of a car.
A noise generated in a car interior, which is an enclosed space, can include engine noise, road noise that depends on the structure of the surface layer of road pavement, or wind noise around doors and windows that is produced during driving. Among these noises, the engine noise, which has a strong correlation with frequency of rotation, occupies a considerable portion of the entire interior noise of car.
In a typical active noise control system for controlling vehicle interior noise, for example as illustrated in FIG. 1, the vehicle interior noise sensed by a microphone 1 is suitably filtered through a filter 2 and is inputted to a controller 5 through an analog-to-digital (A/D) converter 3. Preferably. the controller 5 applies multiple algorithms to the input value to produce a computed result by which the current level of the noise can be adjusted to be minimized. Based on the computed result, a control signal is digital-to-analog converted by using a D/A converter and then is provided to an amplifier 4. Preferably. the control signal, of which level is amplified by the amplifier 4, is outputted through a speaker 6.
In the above-described active noise control apparatus for a vehicle, a noise reduction function is utilized to minimize a car noise in order to reduce an engine booming sound. However, there is also a need to cancel noises other than a noise at a specific RPM range.
The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.